Feldspar is the name given to a group of nonmetallic minerals that are
much alike. Several of them are so similar that a petrographic
microscope must be used to tell them apart. Each of the feldspar
minerals is an aluminum silicate. Each of them contains, in addition, at
least one of the following elements: potassium, sodium, calcium, and
barium. The feldspar minerals that are found in Texas include _albite_,
a sodium-aluminum silicate, and _orthoclase_ and _microcline_, which are
both potassium-aluminum silicates.
The feldspar minerals are transparent to translucent and have either
glassy or pearly lusters. They can be white, cream, or a shade of red,
brown, yellow, blue, gray, or green. When you rub a feldspar across a
streak plate, it leaves a white streak. The feldspars are rather hard—a
pocket knife will not scratch them, although a piece of quartz or a
steel file will. These minerals have good cleavage in two directions.
The cleavages meet at an angle of about 90°, so that the cleavage
fragments have square corners.
[Illustration: Feldspar cleavage fragment from Llano County, Texas. The
two directions of good cleavage meet at an angle of about 90°.]
The feldspars are important rock-forming minerals. You can find them in
igneous rocks, such as granite or pegmatite, and in metamorphic rocks,
such as gneiss. They also occur as fragments in sedimentary rocks, such
as some sandstone and conglomerate.
Although the feldspars can originate in other ways, they form mostly
from hot magmas that cool and crystallize into igneous rocks. These
minerals occur in the rocks as grains, as cleavable masses, and as
individual crystals. The crystals may be shaped like prisms, or they may
be flat and slabby.
Good places to look for feldspars are in areas where granites,
pegmatites, and other intrusive igneous rocks appear at the surface. The
pegmatite rocks of Burnet, Gillespie, Llano, and Mason counties in the
Llano uplift area of central Texas, and those of the Van Horn Mountains
in Hudspeth and Culberson counties in west Texas, are especially good
sources of feldspar. Large cleavable masses and crystals that are more
than a foot long are found in some of these rocks.
The feldspars have a number of uses. Some of the pegmatite feldspars
from Llano County in central Texas have been crushed and used as
granules for built-up and composition roofs. In addition, some have been
shipped to Mexico for glass-making. Some of the other uses of feldspar
are in making porcelain, ceramic glazes, and scouring compounds. A few
of the feldspar minerals, such as the variety of microcline known as
_amazonstone_, are used as gemstones.
[Illustration: Microcline feldspar crystals from near Granite Shoals
Lake, Llano County, Texas.]
Fibrous Gypsum. _See_ Gypsum.
Flint. _See_ Quartz.